Knee arthroplasty is becoming more common to partially or totally replace knee joint components which have been damaged due to trauma or disease. Depending upon the condition of the ligaments and tendons surrounding the joint, a surgeon can select a prosthetic component which provides the necessary degree of stability to the knee joint. Provided the cruciate ligaments are sufficiently stable, a prosthesis can be selected which utilizes the natural soft tissue structures. This approach is preferred since the complex operation of the knee is most difficult to duplicate artificially.
A posterior-stabilized femoral prosthesis is indicated for a patient suffering from an unstable, painful knee joint where the instability is caused by the lack of or inadequacy of the posterior cruciate ligament. The posterior-stabilized knee joint consists of a tibial component having a stabilizing post protruding from the superior surface, or table, of the tibial component, and a femoral component with a posterior stabilizing housing to accept the stabilizing post and thereby provide the stability which the patient's knee joint lacks. Examples of posterior-stabilized total knee joint prosthesis may be had in U.S. Pat. Nos. 4,213,209, to Insall et al, issued Jul. 22, 1980, and 4,298,992, to Burstein et al, issued Nov. 10, 1981.
A mentioned above, if the patient's ligaments are sufficiently stabilized, a conventional condylar-stabilized component is preferably implanted, and the ligaments allowed to remain intact, thus resulting in a more natural and better functioning prosthesis. Even though satisfactory upon initial implantation surgery, the patient's ligaments may deteriorate over time so that they no longer adequately stabilize the artificial knee joint. Under these circumstances, revision surgery is necessary to convert the femoral component and tibial component of the prosthesis to a posterior-stabilized type, such as shown in U.S. Pat. No. 4,714,474, to Brooks, Jr. et al, issued Dec. 22, 1987, and assigned to the assignee of the subject invention, the disclosure of which is hereby expressly incorporated by reference and relied upon.
During revision surgery, the condylar-stabilized type of femoral component must be removed and replaced by a posterior-stabilized femoral component. Often, the femoral component includes an integral stem implanted and fixated within the femur, which stem must be removed from the femur resulting in considerable surgical trauma to the bone. European Patent Application Number 0336774, in the name of Richards Medical Company, published Oct. 11, 1989, discloses a modular femoral component having a detachable stem. Also, the Richards Medical prosthesis includes a modular posterior stabilizing housing which is detachable with respect to the modular stem. The primary disadvantage of the Richards Medical modular prosthesis resides in the manner in which the modular posterior stabilizing housing is attached to the condylar portion of the femoral components. More specifically, a pair of threaded posts extend proximally from the condylar portion and are received through corresponding apertures in the posterior stabilizing housing, such that a threaded portion of the posts extends above the posterior stabilizing housing. A pair of nuts are threadably engaged with the extending threaded portions of the posts and are tightened to fixedly retain the posterior stabilizing housing against the condylar portion.
The manner in which the Richards Medical modular posterior stabilizing housing is attached to the condylar portion presents numerous disadvantages. First, during implantation, the surgeon must resect corresponding portions of the bone to accommodate the two nuts which retain the posterior stabilizing housing to the condylar portion. Further, it is well known that threaded fasteners of the like disclosed by Richards Medical are prone to loosening over time thereby leading to premature failure of the prosthesis. Also, the nuts are prone to improper assembly, wherein the surgeon or other assembling personnel may inadvertently cross thread the nuts upon the threaded posts, thus further increasing the possibility of premature failure of the prosthesis. Also, such threaded fasteners are prone to fracture or other stress related failure due to the inherent non-uniform stress distribution in standard threaded fasteners. Further, the Richards Medical prosthesis is relatively expensive to produce and stock in that several inventory parts are required, i.e., modular posterior stabilizing housings in addition to a plurality of threaded fasteners.